\documentclass[15pt]{article}
\usepackage{CJKutf8}
\usepackage{extarrows}

\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{latexsym}
\usepackage{mhchem}
\usepackage{chemfig}


\pagestyle{empty}

\begin{document}
\begin{CJK*}{UTF8}{gbsn}
\setlength{\parindent}{0pt}
\newcommand{\amsatop}[2]{{\genfrac{}{}{0pt}{1}{#1}{#2}}}
\newcommand{\twolines}[2]{{\amsatop{\mbox{#1}}{\mbox{#2}}}}
\newcommand{\fs}{{5}}
\Large

\noindent
$ #expression# $
\noindent
\end{CJK*}
\end{document}